Dental radiology has undergone important changes over the past several decades. However, the need for more precise diagnostic imaging methods continues to be a high priority. Intraoral dental X-rays were introduced only one year after Roentgen's discovery of X-ray radiation. Since that time, advances in dental imaging techniques have included more sensitive detector technology, panoramic imaging, digital imaging and Cone Beam Computed Tomography (CBCT). Computed Tomography (CT), Magnetic Resonance Imaging (MRI), Ultrasound (US), and optical techniques have also been investigated for dental imaging.
Intraoral radiography is the mainstay of dental imaging. It provides relatively high resolution, and limited field of view images for most routine dental needs. However, as a two dimensional (2D) imaging modality, the technique suffers from superimposition of overlying structures and loss of spatial information in the depth dimension. Panoramic imaging, a popular form of extraoral imaging, visualizes the entire maxilla, mandible, temporo-mandibular joints (TMJ) and associated structures in a single image, but it is subject to considerable geometric distortion and has relatively low spatial resolution compared with intraoral radiography. CBCT as a three dimensional (3D) imaging modality has found wide acceptance in dentistry, especially for surgical planning procedures such as dental implant and orthodontic treatment planning, and evaluation of endodontic and pathological condition. There are, however, several disadvantages associated with CBCT in comparison to 2D radiography: (1) excess noise and artifacts from metal dental restorations/appliances reduce the image quality; (2) acquisition, reconstruction, and interpretation time are greatly increased, reducing clinical efficiency and increasing financial cost; and (3) significantly higher ionizing radiation doses increase radiation burden for the patient.
Despite the many technological advances, the radiographic diagnostic accuracy for some of the most common dental conditions has not improved in many years and in some cases remains low. Examples include caries detection, root fracture detection, and assessment of periodontal bone loss.
Caries is the most common dental disease. The World Health Organizations estimates that 60-90% of school children and nearly all adults have dental caries at some point in time. If carious lesions are detected early enough, i.e. before cavitation, they can be arrested and remineralized by non-surgical means. When carious lesions go undetected, they can evolve into more serious conditions that may require large restorations, endodontic treatment, and, in some cases, extractions. The detection sensitivity of caries has not seen any significant improvement in the past several decades. 2D intraoral radiography is the current gold standard, with a reported sensitivity ranging from 40% to 70% for lesions into dentine and from 30% to 40% for lesions confined to enamel. CBCT does not provide significant improvement for caries detection. Beam-hardening artifacts and patient movement decrease structure sharpness and definition.
The detection of vertical root fractures (VRF) represents a clinically significant diagnostic task with important ramifications in tooth management. VRFs are considered one of the most frustrating tooth conditions associated with endodontic therapy. Overall detection of VRFs remains poor. The ability of CBCT to detect initial small root fractures is limited by its relatively low resolution. Furthermore, excess beam hardening, streak artifact, and noise result in both significantly decreased sensitivity and increased false positive root fracture diagnosis.
Dental radiography provides important information for assessing tooth prognosis and making treatment decisions associated with periodontal disease. Conventional 2D intraoral radiography provides exceptionally high image detail of key dental structures, but because of structure superimposition delivers poor assessment of alveolar bone architecture and consistently underestimates bone loss. CBCT conversely delivers more accurate 3D assessment of clinically-relevant morphologic alveolar bone defects but with a penalty in image detail. Beam hardening and streak artifacts are a significant problem for accurate bone morphology characterization.
These diagnostic tasks illustrate the clinical need for a diagnostic imaging system with high resolution, 3D capabilities, reduced metal artifact and lower radiation burden to patients.
Digital tomosynthesis imaging is a 3D imaging technique that provides reconstruction slice images from a limited-angle series of projection images. Digital tomosynthesis improves the visibility of anatomical structures by reducing visual clutter from overlying normal anatomy. Some examples of current clinical tomosynthesis applications include chest, abdominal, musculoskeletal, and breast imaging.
A variation of the tomosynthesis technique, called Tuned Aperture Computed Tomography (TACT), was investigated in the late 1990's for dental imaging. TACT significantly improved the diagnostic accuracy for a number of tasks compared to conventional radiography. These included: (1) root fracture detection, (2) detection and quantification of periodontal bone loss, (3) implant site assessment, and (4) the evaluation of impacted third molars. The results for caries however were inconclusive.
TACT was not adopted clinically because the technology was not practical for patient imaging. Conventional x-ray tubes are single pixel devices where x-rays are emitted from a fixed point (focal spot). To acquire the multiple projection images, an x-ray source was mechanically moved around the patient. A fiduciary marker was used to determine the imaging geometry. The process was time consuming (e.g., approximately 30 minutes per scan) and required high operator skill to accomplish image acquisition.
Extraoral tomosynthesis has been investigated in a patient study using an experimental device, and using CBCT. The extraoral geometry required high radiation dose. The image quality was compromised by cross-talk of out-of-focus structures. Intraoral tomosynthesis using a single mechanically scanning x-ray source has been described in the patent literature, and investigated in a recent publication using a single conventional x-ray source and a rotating phantom. Unfortunately, the limitations described above for TACT remained the same for these approaches, which are caused primarily by the conventional single focal spot x-ray tube.